Through the use of dynamic and opportunistic spectrum access, cognitive radio (CR) enables high spectrum efficiency. The term cognitive radio was first proposed in late 1990s (see, J. Mitola and G. Q. Maquire, “Cognitive radio: making software radios more personal,” IEEE Personal Communications, August 1999) and a comprehensive overview has been provided in S. Haykin, “Cognitive radio: Brain-empowered wireless communications,” IEEE Journal on Selected Areas in Communications, vol. 23, No. 2, pp. 201-220, February 2005.
The basic concept of CR is to allow unlicensed CR users, also called secondary users, to use licensed spectrum bands (also referred to as licensed frequency bands) as long as they do not cause interference to licensed users, also called primary users. Therefore, CR users must be able to identify and use spectrum bands that are not being used by primary users. In practice, the available spectrum bands for CR users may vary with time and location. A region of time/spectrum/location available for a CR user is called a spectrum hole (SH). In general, a CR system may coexist with a licensed system by transmitting/receiving through SHs.
The SH is the basic resource for CR users. In a report from the Federal Communications Commission (FCC), it was shown that with a very large probability, there are always some licensed spectrum bands that are not used by primary users in a certain location or time (see, Federal Communications Commission, Spectrum Policy Task Force 1/8, an Interleaver Report, ET Docket No. 02-155, Nov. 2, 2002). So far, considerable effort has been undertaken to increase the spectrum efficiency by allowing CR users to exploit these unused bands (see, e.g., D. Cabric, S. M. Mishra, and R. W. Brodersen, “Implementation issues in spectrum sensing for cognitive radios,” Proc. 38th. Asilomar Conference on Signals, Systems, and Computers, vol. 1, November 2004, pp. 772-776; F. F. Digham, M. S. Alouini, and M. K. Simon, “On the Energy Detection of Unknown Signals Over Fading Channels,” IEEE Transactions on Communications, vol. 55, No. 1, pp. 21-24, January 2007; J. Ma and Y. (G.) Li, “Soft Combination and detection for cooperative spectrum sensing in cognitive radio networks,” Proceedings of IEEE GLOBECOM, November 2007, pp. 3139-3143; G. Ganesan and Y. (G.) Li, “Cooperative spectrum sensing in cognitive radio—part I: two user networks,” IEEE Transactions on Communications, vol. 6, No. 6, pp. 2204-2213, June 2007; “Cooperative spectrum sensing in cognitive radio—part II: multiuser networks,” IEEE Transactions on Communications, vol. 6, No. 6, pp. 2214-2222, June 2007).
Spectrum sensing seeks the presence of SHs so that CR users may utilize them. While most of existing work on spectrum sensing focuses on detecting SHs in time and location, the possibility that the CR system coexists with the licensed system at the same location, time, and frequency has not been well investigated.